Continuous anode for electrolytic cells



United States Patent 3,471,383 CONTINUOUS ANODE IFOR ELECTROLYTIC CEL SHerman H. Tiedemann, Scotch Plains, N.J., assignor to GAF Corporation, acorporation of Delaware Filed Feb. 5, 1965, Ser. No. 430,684 Int. Cl.C22d 1/04; C01d 1/08; B01]: 3/08 US. Cl. 20499 9 Claims ABSTRACT OF THEDISCLOSURE A movable web impregnated with carbon is substituted for aconsumable carbon anode in an electrolytic cell.

The present invention relates to an improved continuous anode forelectrolytic cells. It involves use of elongated flexible web materialwhich is particularly suitable for use as anodic material inelectrolytic chlorine caustic cells, but it has uses also in other typesof electrolytic cells.

In the prior art, chlorine and caustic soda have generally been producedby electrolysis of sodium chloride, using massive stationary graphiteanodes and cathodes of mercury and the like. A typical unit, forexample, contains 42 cells, each provided with 144 stationary graphiteanodes which weigh about 65 pounds each. Such a plant has a ratedcapacity of something like 180 tons of chlorine per day. As these cellsare used, the graphite blocks or anodes gradually are consumed due tochemical oxidation. They diminish in thickness as oxidation, especiallychlorination, takes place. It is estimated that about six pounds ofgraphite anodes are used up for every ton of chlorine produced. Graphiteas such is not necessarily or particularly expensive but the stepsnecessary for its replacement frequently may be.

Ultimately, the conventional graphite anodes described above areconsumed to the point where they becom very thin. Their electricalresistance rises and they also become deficient in mechanical strength.Hence, they must be replaced. Replacement with new anodes is anexpensive procedure, not so much due to the cost of the anode per se butmore particularly because the cell which is affected must be taken outof production while the anode is replaced. Replacement also entailsconsiderable labor. In conventional practice, the cell covers areremoved after the system has been shut down. Then a rather tediousoperation of removing the spent anode blocks and replacing them withfresh anode blocks is undertaken. Thereafter the cell must bereassembled and put back into operation. In a typical operation, itrequires about four days to replace the carbonaceous or graphitic anodesin a cell, in some cases, more. During this time, the cell is producingnothing. A number of men, as many as five, are commonly employed in alarge plant continuously for the sole purpose of replacing spent anodesin electrolytic cells.

A particular object of the present invention is to make it possible toeliminate the conventional anode block replacement described above.

It is obvious that efficient operation of an electrolytic cell greatlyaffects the economics of a plant for producing chlorine and causticsoda. The same applies to other plants of comparable design and purpose.In such plants, a typical cell normally operates with an appliedelectrical potential in the range of about 4 to 5 volts. As thestationary graphite blocks mentioned above are gradually depleted, thevoltage in the cell rises. The resistance of each block increases and,ordinarily, in an effort to reduce the voltag drop, the blocks may begradually lowered 3,471,383 Patented Oct. 7, 1969 further into the cell,i.e., closer to the mercury layer. There is a practical limit to such.But any unnecessary voltage drop is very costly. In a typical plant each& of a volt per cell required for operating the entire plant representsan annual cost of $40,000 for electricity.

In addition to the cost of electric power, in conventional practice, twofull-time employees are required to adjust more or less continuously theheight of the anodes in an effort to control the voltage drop a theanodes are depleted. Besides eliminating this labor, the use of acontinuous anode, as is taught by the present invention, permitscontinuous operation of the unit with a steady voltage or with a minimumvoltage drop.

The present invention involves as a particular feature the use of agraphite cloth web as an anode. This Web is a long strip of fabric wellloaded with the carbonaceous material. In'addition to the web, ofcourse, contactors are provided in the apparatus for the efficientdistribution of electric current through the graphite web or cloth. Thearrangement is such that the wear or attrition of the carbonaceoussubstance of the graphite or graphite-loaded cloth is quite uniform. Thecloth or web is moved by suitable simple mechanism, and preferablycontinuously, or at regular short intervals, so as to take care of thedepletion of the graphite substance and keep the cell opearting in auniform manner. With the present arrangement, depletion is very slow,preferably a new graphite loaded cloth is fed in at one end of the celland passed through the cell at such a rate that its useful life orcarbon content for electrolytic purposes is fairly well spent by thetime it emerges from the cell. By suitable arrangement of electriccontacts, full use is obtained from the cloth and most of thedifficulties described above are completely avoided.

The arrangement of the present invention preferably is such that a longweb of cloth or fabric anode either fabricated largely of graphite, orimpregnated with graphite to a suitable degree, is fed into the cell atone end and out the other end at such a rate that it is substantiallycompletely consumed. In some cases the web may be fed through the celland back near the point of entrance before it leaves the cell. Thearrangement preferably is such that the voltage rise to compensate forchanges in conductivity is minimal. The invention contemplates that thespent carbon bearing web or anode preferably be continuously reeled outof the cell for discard, although as noted above the movement may bediscontinuous provided it is made at frequent intervals.

According to the present invention, the continuous anode web may bepassed through either a horizontal cell or a vertical cell, both typesbeing well known. In general, the same principles are involved in bothsituations.

The invention will be more fully understood by referring to the drawingswhich form a part of this specification.

In said drawings, FIGURE 1 is a diagrammatic view of a typical prior artcell in vertical section showing the conventional graphite anodes in theform of blocks suspended into the cell.

FIGURE 2 shows the concept of the present invention as applied to acontinuous flexible web anode in a horizontal cell. This view isgenerally like that of FIG- URE 1, the same basic cell arrangement beinginvolved but the stationary block anodes are replaced with a continuousflexible web of suitable conductive carbon bearing or graphitic cloth.

FIGURE 3 shows a vertical type cell in which the principles of theinvention are involved, this being a vertical sectional view.

FIGURE 4 illustrates a variation in the system of FIGURE 2.

Referring in greater detail to the drawings, FIGURE 1 shows a basiclayout for a typical mercury type chlorinecaustic cell. Anodes 13, ofwhich four are shown, are suspended from the ceiling of the cell in sucha manner that they can be lowered, i.e., their height may be adjusted.Suspending means are not shown except that each block has an upwardlyextending column passing through the ceiling 15 of the cell. Electriccurrent is supplied from a source 19, distributed through severalconductors 21 to the several anodes. Cathode contactor points or members25 are shown resting on the floor 27 of the cell, these beingelectrically connected through suitable conductors 29 to the negativeside of the current supply 31.

The cathodic points 25, which are in multiple and preferably in aboutthe same number as the anode blocks 11, supply current to the cathodiclayer 33 of mercury. The latter flows through an inlet 35 into the celland moves to the left along the fioor of the cell in a horizontal layer33 to outlet 37. From the latter, the mercury which by now is an amalgamwith sodium is taken to the denuder for mercury recovery. The recoveredmercury is then recycled through inlet 35. The same system obviously isapplicable to treatment of other salts, especially other alkali metalhalides besides sodium chloride.

The strong brine of the salt being electrolyzed is supplied throughinlet line 41, shown at the right of the cell. It flows along the cellfrom right to left as electrolysis takes place. Gaseous chlorine isevolved from the top of the cell through outlet 43 and the spent brinepasses out through outlet 45. The sodium of course is convertedprimarily to NaOH.

As pointed out above, the conventional graphite anodes 11 are rapidlyconsumed. In a commercial cell, the number of anodes may varyconsiderably. Four only are shown in FIGURE 1, but in a commercial cellthe number usually is much greater.

By comparison, the arrangement of the present invention for a horizontalcell is shown in FIGURE 2. Herethe body of the cell is much the same asthat of FIGURE 1, mercury being supplied through inlet 35a and themercury-sodium amalgam passing out through outlet 37a. Electric currentis supplied from the positive source 19a and negative source 31a. Anodepoints 25a are the same as in FIGURE 1 in general arrangement.

Instead of supplying current from the positive leads to graphitic anodesas in FIGURE 1, in this case these leads are attached to conductiveroller members 61 through leads 63. These rollers contact the conductivecarbonaceous or graphite-loaded fabric 65, which is a key element in thepresent invention. This web is fed from a supply roll 67 through anopening 69 in the top of the cell and thence around a guide roller 71and underneath the contact rollers 61 to a second guide roller 73. Therate of movement of the web, by drive means of conventional type notshown, is adjusted so that the carbon or graphite is just about fullyconsumed as the web emerges from the bath. As the web passes aroundroller 73 the graphite in the web has been quite fully consumed. Thespent web then passes upwardly from guide roller 73 through an outletopening 75 in the top of the cell. Outlet 75 is designed to preventsubstantial chlorine gas leakage. It is rewound on take-up roller 77. Itmay be discarded or in many cases it can be impregnated with carbon forreuse.

The mercury layer 83 in FIGURE 2 is essentially the same as layer 33 inFIGURE 1. In both cases the anodes must be kept above and out of contactwith the mercury layer.

As in the case of FIGURE 1, fresh brine flows in through inlet -85 onthe right and the spent brine emerges through outlet 87 at the left.Chlorine gas passes out through the outlet 89 to a suitable tank orstorage system.

Referring next to FIGURE 3, there is shown a vertical type cell havingwalls 101 and 103 which enclose the brine being electrolyzed. The strongbrine enters through inlet 105 and the weak spent brine emerges throughoutlet 107. In this case a continuous mercury film, which serves as thecathode, is obtained by spilling the denuded mercury entering at the topthrough inlet 111 down over a vertical cathodic grid. The latter isindicated diagrammatically at 113. In its passage down the grid, themercury is converted to the sodium amalgam. At the bottom of the cell,the amalgam is pumped off through outlet 113 to the denuder system whichis conventional and forms no part of the present invention. Thecontinuous anodic web of graphite cloth is supplied from a reel 115. Itenters at the top of the cell through an inlet 117. This inlet is narrowenough to prevent any substantial loss of chloride gas. The web 119passes downwardly around a pair of guide rollers 121 and 123. From thelatter the web passes upwardly through the vessel and thence throughoutlet 125 and around guide roller 127 to a take-up reel 129. The spentweb is rewound on the latter and can be discarded or subsequentlycarbonized or graphited anew to serve as a new anode.

In some respects the mechanical features involved in reeling the cloththrough the vertical cell are simpler than those in the horizontal cell.There is no long horizontal run to support out of contact with a mercurylayer. Current is supplied to the web through suitable electriccontactors such as the rollers 127 and 121 and 123. The connections arenot shown but would be obvious to those skilled in the art. In addition,supplemental contact rollers may be positioned along the path of thegraphite web, along the incoming downward traveling stretch if desiredand particularly along the rising stretch. Sufficiently conductivecontacts must be supplied to give effective electrolytic current to thecell. These are omitted from FIGURE 3 in the interest of simplicity,their nature and number being dependent obviously on the capacity of thecell and its size, as will be apparent to those skilled in the art.

Normally, in the arrangement of FIGURE 2, the web will be buoyed upagainst the contacting rollers 61 sulficiently that no additionalsupport means are required. Under some circumstances, however, anddepending on the length of the web, the tension applied thereto and thespacing of the contact roller 61, additional support may be needed. Anarrangement for this purpose is shown in FIGURE 4.

In FIGURE 4 it will be understood that the web is essentially the sameas web 65 in FIGURE 2, and its arrangement and functioning are similar.The only difference is that additional support rollers 167 are providedin the bath. These must be kept above the mercury level to avoidshorting out the cell, and therefore they are preferably small rollers.Moreover, they may be raised or lowered, as indicated in the dottedlines, to cause the web to make better electrical contact, that is, towrap more effectively around the guide rollers 161, which correspond inother respects to roller 61 of FIGURE 2. This arrangement affords betterelectrical contact between the web and the main guide rollers. Ifdesired, the rollers 167 may be raised until their lower surfaces areessentially in line with the lower surfaces of guide rollers 161, aswell as rollers 171 and 173. The latter correspond to the rollers 71 and73 of FIGURE 2. This arrangement may be desirable or necessary in somecases to avoid shorting the cell by the rollers being too close to themercury layer.

Graphite cloth is a relatively new product and is preferred for thepurposes of the present invention. It is possible, however, to use othercarbonaceous webs or carbonloaded webs of various fabrics foraccomplishing the same general purposes. The web material must be onewhich will not distintegrate in the bath. It will be app r eciated thatthe present invention eliminates many problems connected with the priorart practices.

It will be understood that various mechanical arrangements andmodifications other than those shown and described above may be made, aswill be obvious to those skilled in the art. Essentially, the systemrequires maintenance of good electrical contact with the fabric web.

It requires that the web itself have a good content of conductivegraphite. It is also essential, of course, that it be kept out ofcontact with the mercury layer, but properly spaced therefrom so as toobtain maximum operating efiiciency at optimum voltage and currentvalues.

It is intended by the claims which follow to cover the variations andmodifications in the system which would occur to those skilled in theart.

What is claimed is:

1. The method of operating an electrolytic cell having a mercury cathodefor production of a halogen from a brine, said cell being provided witha traveling anode comprising an elongated web of graphite-loaded fabriccomprising passing said anode into and through said brine at such a ratethat said graphite is substantially completely consumed when said webemerges from the brine.

2. .The method of claim 1 in which said elongated web contacts aplurality of conductive roller members which are connected to a sourceof positive electric current.

3. The method of claim 2 in which said graphite diminishes in proportionto distance that said web has moved through said brine when said rate oftravel is constant.

4. The method of claim 3 in which the distance between the surfaces ofsaid web and of said mercury cathode is varied to maintain maximumefiiciency at optimum voltage and current values.

5. The method of claim 4 in which said mercury cathode is horizontallydisposed along the bottom of said electrolytic cell and said web ismaintained at a selected distance thereabove so that voltage rise, whichcompensates for changes in conductivity of said traveling anode withprogressive consumption of graphite, is minimal.

6. The method of claim 2 in which said electrolytic cell is a verticaltype cell.

7. The method of claim 6 in which said mercury cathode is a continuousmercury film which is obtained by spilling denuded mercury, entering atthe top of said electrolytic cell, over a vertical cathodic grid.

8. The method of claim 4 in which said fabric of said elongated web isloaded again With graphite for re-use.

9. The method of claim 4 in which said web moves discontinuously inshort increments of travel.

, References Cited UNITED STATES PATENTS 1,750,331 3/1930 Carns 204-2072,323,042 6/1943 Honsberg 204 220 2,933,433 4/1960 Lancy 204 2062,953,507 9/1960 Palme 204 206 3,244,612 4/196 Murphy 204-294 FOREIGNPATENTS 24,303 2/1896 Great Britain.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US.or. X.R. 204206, 219, 294

